Field of the Invention
The invention relates generally to navigation systems and, more particularly, to navigation systems that incorporate inertial and GNSS subsystems.
Background Information
Inertial/GNSS receivers, such as the receivers described in U.S. Pat. Nos. 6,721,657 and 7,193,559, which are assigned to a common assignee and incorporated herein by reference, work well to provide accurate and uninterrupted navigation information, even in environments in which sufficient numbers of GNSS satellites are not continuously in view. As is described in the patents, the inertial/GNSS receivers utilize inertial measurements to fill-in whenever the GNSS subsystem does not receive GNSS satellite signals from a sufficient number of GNSS satellites to determine position. Further, the inertial/GNSS receivers combine, in real time, information from the GNSS and inertial subsystems to aid in signal re-acquisition and in the resolution of associated carrier ambiguities when a sufficient number of GNSS satellite signals are again available.
The inertial/GNSS receivers initialize inertial and GNSS subsystems at start-up and the inertial/GNSS receiver can then operate in steady state navigation mode to provide accurate and uninterrupted navigation information to a user. The inertial subsystem must typically experience dynamic motion both during and after start-up in order for the inertial/GNSS receivers to accurately calculate the navigation information utilizing a combination of inertial measurements, GNSS observables, and GNSS position and covariance information.
The inertial sub-system includes an inertial measurement unit (IMU), which reads data from orthogonally positioned accelerometers and gyroscopes. As is known, the accelerometers and gyroscopes have associated biases that introduce errors into the IMU data if not corrected. The inertial sub-system thus incorporates GNSS position, covariance and, as appropriate GNSS observables in an INS filter to estimate the IMU errors required to correct the INS measurements.
The gyroscopes in the IMU, particularly in a relatively low cost IMU, tend to have very large biases that can drift quickly when left un-aided. Accordingly, the uncorrected gyroscope measurements can cause inaccurate heading or attitude information, which will introduce errors into the calculations of position and velocity. Known prior systems utilize course over ground measurements or, alternatively, absolute orientation, which is determined in a known manner using signals from multiple GNSS antennas, to provide updated heading information. The system uses the updated heading information with RTK in a known manner to correct for carrier phase errors caused by, for example, phase windup. The GNSS/INS system then uses the corrected carrier phase to determine improved position and velocity, and provides the improved position and velocity to the IMU in order to constrain IMU device drift.
The two techniques for determining the updated heading information for the correction of the carrier phase errors typically introduce other errors into the position and velocity calculations. The errors, which are generally associated with unknown crab angles and a dependence on motion, must then be handled by the system in order for the system to calculate accurate position and velocity information using the IMU sensors.